Solar electric power generation systems, particularly those based on photovoltaic solar panels, continue to gain popularity in efforts to shift away from supply-limited, greenhouse-gas producing fossil fuels to more environmentally friendly and sustainable forms of energy. Most of today's conventional rooftop photovoltaic solar panels comprise side-by-side arrangements of relatively large (e.g., 5 cm×5 cm, 10 cm×10 cm) bulk monocrystalline or bulk multicrystalline silicon wafers processed to form depthwise p-n junctions. Manufacture of the bulk crystalline silicon wafers is highly energy-intensive and expensive, and the power conversion efficiency of the resultant devices is typically only in the range of 15% to 20%. A chronic shortage in the supply of bulk crystalline silicon wafers has plagued the industry in recent years, a shortage that is expected by some forecasters to reach crisis proportions in coming years. Although thin-film photovoltaic cells fabricated from amorphous silicon or chalcogenide compounds require less semiconductor material than those based on bulk crystalline silicon wafers and are less energy-intensive and less costly to produce, their power conversion efficiencies are even lower, usually in the 6% to 10% range.
Proposals have been set forth for using semiconducting nanowires as a basis for photovoltaic solar energy conversion. Nanowires are small self-assembled structures having lengths typically in the range of 0.5 μm-5 μm and diameters typically in the range of 10 nm-500 nm. One method of fabricating nanowires uses a vapor-liquid-solid (“VLS”) synthesis process, sometimes termed a catalytic growth process. A catalyst material such as gold or titanium is deposited on a substrate at a large number of spots thereacross, each spot being a location at which a nanowire will be grown. The substrate with the catalyst is then placed in a reaction chamber and heated to high temperatures (e.g., 250° C.-1000° C.). Precursor gases, including the elements or compounds that will form the nanowires, are introduced into the chamber. Under the influence of the catalyst, the precursor gases at least partially decompose into their respective elements, some of which are transported on or through the catalyst in liquified phase to the immediately underlying solid surface provided by the substrate. At each spot, a nanowire epitaxially grows outwardly from the substrate as the process proceeds, the catalyst at each spot remaining at the tip of the nanowire and rising away from surface of the substrate as the nanowire grows. The resultant nanowires exhibit a long-range atomic order (i.e., single-crystal) that can potentially be exploited for a variety of different useful applications. So-called self-catalytic growth of nanowires has also been reported in the literature.
During nanowire formation, the elements or compounds used to form the nanowires can be varied, such that the material composition and/or semiconductor doping level of each nanowire is variable along the longitudinal length of that nanowire. In one known scenario relevant to the preferred embodiments herein, the longitudinally varying material and/or doping profile can be designed such that each nanowire exhibits a photovoltaic property, i.e., is capable of absorbing incident photons and providing an associated photocurrent to an external load (if properly electrically connected to that load). As used herein, PV nanowire refers to any of a variety of nanowires themselves or related structures that employ nanowires and are capable of exhibiting photovoltaic properties, such photovoltaic properties arising from any of a variety of different material selections, material compositional and spatial chemical profiles, and/or doping profiles thereof. By way of non-limiting example, PV nanowires can comprise one or more longitudinal homojunctions (e.g., p-n, p-i-n, p-n-p, n-p-n homojunctions), one or more longitudinal heterojunctions (e.g., materials containing various chemical elements and/or various chemical compositions), and/or portions of such homojunctions or heterojunctions that are completed by virtue of the materials and/or doping profiles immediately opposite the longitudinal end(s) of the nanowire. One example of a PV nanowire-based solar cell is described in US 2007/0267625 A1, which is incorporated by reference herein.
Advantageously, PV nanowire-based solar cells can provide power conversion efficiencies that are as great, or even greater, than solar cells based on bulk crystalline wafers made of a same material. For example, it is believed at least theoretically possible to achieve PV nanowire-based solar cells composed of III-V semiconductor material having 35 percent, and perhaps even 40 percent, energy conversion efficiency, which is as high or better than photovoltaic cells made with bulk crystalline semiconductor wafers. At the same time, because they can be built upon low-cost substrates with low material utilization and comparatively low energy requirements, PV nanowire-based solar cells can be fabricated at a fraction of the cost of comparable solar cells based on bulk crystalline wafers.
One or more issues arises in the design and/or fabrication of PV nanowire-based solar cells that is at least partially resolved by one or more of the preferred embodiments described herein. One issue that arises in PV nanowire-based solar cells relates to the need for an electrode to be provided on each side of the PV nanowire array, including a root-side electrode corresponding to the roots of the PV nanowires and a tip-side electrode corresponding to the tips of the PV nanowires. The need to provide good electrical contact and conductivity at these electrodes can present substantial limitations on the type, complexity, and orientation of the overall PV nanowire-based solar cells, because structures providing the good electrical contact and conductivity can often prove to be light-absorbing or light-reflecting, thereby reducing the percentage of photons able to reach the PV nanowire junctions. Another issue that can arise in PV nanowire-based solar cells relates to the material content, other than the PV nanowires themselves, of the space between the root-side electrode and tip-side electrode. In US 2007/0267625 A1, supra, a layer of insulating material is provided that fully occupies the space (other than the PV nanowires themselves) between the root-side electrode and tip-side electrode. Although the layer of insulator material can serve a useful function as a general support for the tip-side electrode as well as lateral support for the PV nanowires, it can possibly be disadvantageous over the long run, in that it may physically and/or chemically degrade with long-term solar exposure. By virtue of this degradation, the insulator material could “darken,” absorbing larger percentages of photons, dissipating that energy into heat, and reducing the power efficiency of the solar cell. Electrical shorts or efficiency-draining hot spots might also result from the degradation of the insulator material.
One issue that can affect the performance of PV nanowire-based solar cells is carrier loss due to charge recombination occurring along the surface of the PV nanowires. Charge recombination is a loss process in which an electron, which has been photo-excited from the valence band to the conduction band of a semiconductor, falls back into an empty state (hole) in the valence band. Charges that recombine do not produce any photocurrent and, hence, do not contribute toward solar cell efficiency. Charge recombination losses can be particularly strong along the surface of a semiconductor material, where dangling bonds give rise to certain surface states that greatly facilitate the electron-hole recombination process. Because the recombination losses associated with surfaces scale with total surface area, the problem becomes particularly amplified for nanowires having high surface-to-volume ratio. Generally speaking, the recombination losses at surfaces tend to be more problematic for III-V compound semiconductor PV nanowires than for silicon PV nanowires for which stable natural oxide (i.e., SiO2) that can be easily formed on Si surfaces can greatly reduce the density of surface states. Other issues arise as would be apparent to one skilled in the art upon reading the present disclosure.